1. Field of the Invention
The present invention relates to a viscous coupling utilizing viscous fluid, useful for a power transmission system of a vehicle.
2. Description of the Background Art
One conventional viscous coupling is shown in FIG. 1, which can be found in `Automobile Engineering` Jun., 1987 Edition, published by Tetsudo-Nippon Co.
This viscous coupling of FIG. 1 is one to be used in a four wheel drive vehicle of front engine front drive type (FF type), instead of a rear wheel differential device. Thus, there is a driving pinion gear 101 to which torque from an engine is transmitted through a propeller shaft. This driving pinion gear 101 is engaged with a ring gear 102 attached on a diff case 103. Inside the diff case 103, there is a first and a second hubs 104 and 105 which are relatively coaxially rotatable, and which are connected to left and right rear wheel driving shafts 106a and 106b, respectively. The diff case 103 and the first and second hubs 104 and 105 form an operation chamber 107 in which viscous fluid is to be confined. Inside this operation chamber 107 there are plurality of first left and right resistive plates 108a and 108b which are spline connected to the diff case 103 over the first and second hubs 104 and 105, respectively. In addition, there are plurality of second left and right resistive plates 109a and 109b which are spline connected to the first and second hubs 104 and 105, respectively, such that each of the second resistive plate is between two of the adjacent first resistive plates. The operation chamber 107 is divided into first and second chambers 111 and 112 over the first and second hubs 104 and 105, respectively, by a divider 110 in a middle which has a hole 113 connecting the first and second chambers 111 and 112.
Now, when a vehicle is running on a high friction road, the torque from the engine is transmitted to a front wheel driving shaft. In this case, there is no difference in numbers of rotations between the front wheels and rear wheels so that the viscous coupling between the propeller shaft and the rear wheel driving shafts 106a and 106b does not operates and the vehicle runs in front wheel drive mode.
On the other hand, in a situation where the front wheels skid on a low friction road, there appears a large difference between numbers of rotations of the front and rear wheels. In such a situation, the diff case 103 connected to the front wheel driving shaft rotates faster than the left and right rear wheel driving shafts 106a and 106b, so that there is a relative rotation between the first left and right resistive plates 108a and 108b connected to the diff case 103 and the second left and right resistive plates 109a and 109b connected to the left and right rear wheel driving shafts 106a and 106b, which in turn shear through the viscous fluid inside the operation chamber 107. Resulting torque due to shear force of the viscous fluid is transmitted to the left and right rear wheel driving shafts 106a and 106b as an escaping power from skidding of the vehicle.
Similarly, when the left rear wheel skids, there appears a difference in the numbers of rotations between the diff case 103 and the second hub 105 connected to the right rear wheel driving shaft 106b, and consequently there is a relative rotation between the first resistive plates 108b connected to the diff case 103 and the second resistive plates 109b connected to the right rear wheel driving shaft 106b, which in turn shear through the viscous fluid inside the second operation chamber 112. Resulting torque due to shear force of the viscous fluid is transmitted to the right rear wheel driving shaft 106b as an escaping power from skidding of the left rear wheel of the vehicle.
Here, however, in a conventional viscous coupling of FIG. 1, the first right resistive plate 108b and the second right resistive plate 109b in the second chamber 112 relatively rotate, i.e., with respect to each other, whereas the first left resistive plate 108a and the second left resistive plate 109a in the first chamber 109b do not relatively rotate very much. Thus, the relative rotation of the first right resistive plate 108b and the second right resistive plate 109b in the second chamber 112 causes thermal expansion of the viscous fluid in the second chamber 112 which subsequently flows into the first chamber 111 through the hole 113. This in turn makes the viscous fluid in the second chamber 112 less viscous and less condensed so that the torque due to the shear force resulting from shearing of the first right resistive plate 108b and the second right resistive plate 109b through the viscous fluid decreases and no hump phenomena occurs. The hump phenomena is a rapid increase of the transmitted torque occurring above certain inner pressure at which the resistive plates are bound together, as the inner pressure increases by heat generated by the fluid friction of the viscous fluid against the resistive plates, after some consecutive relative motion of the resistive plates.
Consequently, with a conventional viscous coupling of FIG. 1 it is not possible to escape from skidding of one of the rear wheels smoothly, so that the running stability of the vehicle is limited.